GoodKarma Posted October 18, 2019 Report Share Posted October 18, 2019 Hello I've seen quite a few mugs where they only glaze the top half of the mug... I was just wondering if there are negatives to this, such as stains/moisture/mold... to the bottom half? Link to comment Share on other sites More sharing options...
liambesaw Posted October 18, 2019 Report Share Posted October 18, 2019 Only weakness is slight structural weakness and +5 damage to sponges. I do a lot of my bowls this way for extra grip and tactile texture Link to comment Share on other sites More sharing options...
Hulk Posted October 18, 2019 Report Share Posted October 18, 2019 Hi Good! Potential for stain and/or mold greater for clays with high(er) absorption (at full maturity); darker clays may not show staining as much... Probably a bad idea to let any unglazed piece that has high absorption soak in dishwater, whether it's just a bare foot ring or a lot more, as in your pic. Wash, dry. Another concern may be lower strength, particularly at the glazed/unglazed transition. Where the glaze ends at a buttressed point - at the foot ring or opposite the transition arc bottom to wall - lower strength less of a concern? If I were planning to leave a large patch of bare clay, slightly thicker clay at that transition, and a thorough burnishing of the surface as well. What constitutes "low absorption" rate then? ...oh, probably less than two percent? Aardvark SBF is rated 1.5% - bare and burnished, it's almost like matte finish glass... Link to comment Share on other sites More sharing options...
GoodKarma Posted October 18, 2019 Author Report Share Posted October 18, 2019 Thanks! I like the look of it, just wasn't sure if it was still food safe, etc. Link to comment Share on other sites More sharing options...
Min Posted October 18, 2019 Report Share Posted October 18, 2019 @GoodKarma, I would really suggest testing the claybody and the glazes you are thinking of using before doing this. To prevent staining look for a body that has a low absorption figure (1 1/2% or less) and then test it in your kiln with your firing methods and measure what absorption figure you get. If this checks out okay then make sure the clay can take the added stress of having only one side of the pot glazed. To do this take a thin walled cylinder and glaze the inside only with the glaze you are thinking of using as the liner glaze and glaze it as thickly as you would an actual pot. Freeze the test cylinder(s) then put them in the sink and pour boiling water in them. If you will be doing some glaze on the outside then do a test cylinder like this also. If the clay is vitrified then it will be food safe but chances are you won't be eating off the outside of a pot so no worries there. Link to comment Share on other sites More sharing options...
Benzine Posted October 18, 2019 Report Share Posted October 18, 2019 4 hours ago, liambesaw said: Only weakness is slight structural weakness and +5 damage to sponges. I do a lot of my bowls this way for extra grip and tactile texture What if the sponge uses a Spell of Resiliency? Link to comment Share on other sites More sharing options...
liambesaw Posted October 18, 2019 Report Share Posted October 18, 2019 1 minute ago, Benzine said: What if the sponge uses a Spell of Resiliency? It rolls a 2 and fails. Link to comment Share on other sites More sharing options...
Benzine Posted October 18, 2019 Report Share Posted October 18, 2019 4 minutes ago, liambesaw said: It rolls a 2 and fails. Ha! Link to comment Share on other sites More sharing options...
Babs Posted October 18, 2019 Report Share Posted October 18, 2019 Wondering now how old is the oldest handmade mug lasts in the " normal" household with daily use.. Link to comment Share on other sites More sharing options...
Callie Beller Diesel Posted October 19, 2019 Report Share Posted October 19, 2019 @liambesaw @Benzine I've found that if you apply Sherril's Scarlet Kidney of Shining to the projected exposed parts at leather hard, sponges gain a +5 resistance to shredding damage. Your gaming table still will still likely appreciate a coaster. For those non-Dungeons and Dragons players, burnish the exposed part with a little red rib to smooth it out if you're worried about causing damage to sponges or tabletops. Work clean to keep the feet clear of burrs and crumbs, and give your pots a quick pass with some 220 grit wet/dry sandpaper, used wet, after the glaze firing. My clay is fired to around 1% porosity or less, so I have no trouble leaving exposed clay on the bottom 1/4 to 1/3 of the pot. I haven't tried to test where the line is when a mug will break more readily if too much is left unglazed. I know if it's only lined, it's a lot more fragile. Link to comment Share on other sites More sharing options...
liambesaw Posted October 19, 2019 Report Share Posted October 19, 2019 20 hours ago, Babs said: Wondering now how old is the oldest handmade mug lasts in the " normal" household with daily use.. 4000ish years I suppose http://www.mugs.coffee/coffee-mug-knowledge/oldest-coffee-mugs/ Link to comment Share on other sites More sharing options...
Rae Reich Posted October 20, 2019 Report Share Posted October 20, 2019 On 10/19/2019 at 10:02 AM, Callie Beller Diesel said: @liambesaw @Benzine I've found that if you apply Sherril's Scarlet Kidney of Shining to the projected exposed parts at leather hard, sponges gain a +5 resistance to shredding damage. Your gaming table still will still likely appreciate a coaster. For those non-Dungeons and Dragons players, burnish the exposed part with a little red rib to smooth it out if you're worried about causing damage to sponges or tabletops. Work clean to keep the feet clear of burrs and crumbs, and give your pots a quick pass with some 220 grit wet/dry sandpaper, used wet, after the glaze firing. My clay is fired to around 1% porosity or less, so I have no trouble leaving exposed clay on the bottom 1/4 to 1/3 of the pot. I haven't tried to test where the line is when a mug will break more readily if too much is left unglazed. I know if it's only lined, it's a lot more fragile. Delicious!! Link to comment Share on other sites More sharing options...
Rae Reich Posted October 20, 2019 Report Share Posted October 20, 2019 I bought a porcelain mug with a lovely frosty blue glaze and an exposed 1 1/2" bottom area. First use with Lemon Zinger tea revealed a hairline crack going up the exposed area. I suspect that the thickness of the wall as well as the tension between glazed and unglazed both contributed to the fail, but also the relative density of porcelain Exposed porcelain will never really look pristine again after it leaves the kiln. Granite stoneware still looks fine after 20+ years. Most red stone wares also still look good with no staining. Link to comment Share on other sites More sharing options...
Benzine Posted October 21, 2019 Report Share Posted October 21, 2019 On 10/19/2019 at 12:25 PM, liambesaw said: 4000ish years I suppose http://www.mugs.coffee/coffee-mug-knowledge/oldest-coffee-mugs/ I'm guessing it was first used by a manager, to slowly sip, while critically looking at their employees, "Yeah, I'm going to need you, to get those manuscripts turned in, before you leave today..." Link to comment Share on other sites More sharing options...
Babs Posted October 21, 2019 Report Share Posted October 21, 2019 On 10/22/2019 at 3:13 AM, Benzine said: I'm guessing it was first used by a manager, to slowly sip, while critically looking at their employees, "Yeah, I'm going to need you, to get those manuscripts turned in, before you leave today..." And chiselling on rock or making firinf a clay tablet and then mining the ink stone makes for a big day at work. Certainly need that cuppa Benzine of bizarre brain..ah you teach teenager's art classes.. Link to comment Share on other sites More sharing options...
liambesaw Posted October 22, 2019 Report Share Posted October 22, 2019 Yeah benzene you weirdo Link to comment Share on other sites More sharing options...
Melissa F. Posted November 4, 2019 Report Share Posted November 4, 2019 I glaze most of my cups and mugs like this with no issues at all so far. Even put them in the dishwasher and so far so good. My question is though, I made some bottles in this manner but did not glaze the inside. Only 3/4 of the outside. I used laguna 50 stoneware fired to cone 6. Is it still foodsafe to be used as say, an olive oil bottle? And will it be vitrified? I’ve looked around and can’t seem to find a straight answer Link to comment Share on other sites More sharing options...
liambesaw Posted November 4, 2019 Report Share Posted November 4, 2019 IIRC Laguna 50 has 1% absorption @ cone 5, if that's correct it should be fine. Link to comment Share on other sites More sharing options...
Min Posted November 4, 2019 Report Share Posted November 4, 2019 1 hour ago, Melissa F. said: I used laguna 50 stoneware fired to cone 6. Is it still foodsafe to be used as say, an olive oil bottle? And will it be vitrified? I’ve looked around and can’t seem to find a straight answer Is this the clay you used? http://www.lagunaclay.com/clays/northeastern/wc606.php If so then no I wouldn't use it for an oil bottle. Either way you need to do your own testing and not go solely on what the clay companies post as the absorption figures. Your kiln, your pots, your firing schedule etc are all going to make a difference. Clay absorption test in this link, about 2/3 the way down the page. I would add an 8 hour simmer to the test then turn off the stove and leave the sample piece(s) in the water for a total of 24 hours then continue with the drying and weighing. I'm not fond of the idea of leaving an oil bottle unglazed on the inside, it will be very hard to clean should you have oil go rancid inside. Link to comment Share on other sites More sharing options...
liambesaw Posted November 4, 2019 Report Share Posted November 4, 2019 Whoops, I was thinking of #65, sorry about that. Yeah, 3% absorption would get nasty quick. Any reason you want an unglazed interior? Link to comment Share on other sites More sharing options...
Melissa F. Posted November 5, 2019 Report Share Posted November 5, 2019 15 hours ago, liambesaw said: Whoops, I was thinking of #65, sorry about that. Yeah, 3% absorption would get nasty quick. Any reason you want an unglazed interior? I just didn’t have enough glaze to do the interior. I might re-fire now with clear on the inside if I can get my hands on some! Link to comment Share on other sites More sharing options...
Rae Reich Posted November 5, 2019 Report Share Posted November 5, 2019 5 hours ago, Melissa F. said: I just didn’t have enough glaze to do the interior. I might re-fire now with clear on the inside if I can get my hands on some! If the bottle is used to contain oil, I don't think a refire with glaze would work, the glaze can't penetrate the clay enough to be a good coating. What's the point of testing for absorption with water when you intend to use oil? I learned from my mentor that oil lamps, even when glazed with a reliable liner glaze inside, will seep oil through the unglazed base. We brushed two coats of slightly thinned white glue on the bottoms and that sealed against the oil seepage. It dries waterproof, if it isn't soaked in water for extended periods. However, we can assume that the lamp oil still penetrated the liner glaze and remained harmlessly in the body of the piece. For food grade oil, I would not store it, even in any fully glazed ceramic piece, because of the likelihood of oils remaining in the body of the piece and becoming rancid. Possibly for short-term usage, like serving. . Put a flower in that bottle, it's a vase! Link to comment Share on other sites More sharing options...
Min Posted November 5, 2019 Report Share Posted November 5, 2019 44 minutes ago, Rae Reich said: What's the point of testing for absorption with water when you intend to use oil? I think it's important to do the absorption test with water not to rule the pot as tight for oil but to rule out pots that will probably leak. It isn't possible to do a boil and soak test with lamp oil for example but if you know the absorption is much more than zero with water it will probably leak. I don't have issues with my veg/olive oil bottles weeping and the porosity/absorption of my clay is around 1%, it isn't tight enough for lamp oil. Link to comment Share on other sites More sharing options...
Rae Reich Posted November 5, 2019 Report Share Posted November 5, 2019 16 minutes ago, Min said: I think it's important to do the absorption test with water not to rule the pot as tight for oil but to rule out pots that will probably leak. It isn't possible to do a boil and soak test with lamp oil for example but if you know the absorption is much more than zero with water it will probably leak. I don't have issues with my veg/olive oil bottles weeping and the porosity/absorption of my clay is around 1%, it isn't tight enough for lamp oil. @Min Agreed that is a good reason to test with water, too. Not sure I'd want to boil lamp oil, but that should work for food oils. Maybe the volatiles in lamp oil make it more prone to seepage. What do you use for a liner glaze? Cone 10? Link to comment Share on other sites More sharing options...
Min Posted November 5, 2019 Report Share Posted November 5, 2019 53 minutes ago, Rae Reich said: @Min Agreed that is a good reason to test with water, too. Not sure I'd want to boil lamp oil, but that should work for food oils. Maybe the volatiles in lamp oil make it more prone to seepage. What do you use for a liner glaze? Cone 10? Yeah, I think oil lamps are one of the most problematic things to have not leak. I think it has to do with the polarity of the lamp oil molecules versus that of water molecules or veg oils. I remember about 20 years ago there seemed to be a lot of potters making oil lamps and running into problems with weeping. All sorts of sealers were tried then it seems we just moved on from making them. Too much liability. I've made oil bottles at ^10 and 6, for ^10 I used Robins Clear and for ^6 I use my low COE clear. I don't think the glaze is hugely important as the oil molecules will find their way through any glaze flaws or pinholes over time, can't see any flaws that will be inside the bottles. I think the clay itself has to be able to resist the oil from weeping through. Link to comment Share on other sites More sharing options...
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